The present invention is with respect to a kiln, more specially a roller kiln, made up of a preheating zone, a firing zone and a cooling zone, the structures to be fired, more specially ceramic tiles, being moved in a single layer through the kiln without using burning tools of refractory material.
In a roller conveyor kilns designed for firing ceramic material in a single layer and without any burning tools of refractory material, the ceramic material to be fired makes its way through the preheating, firing and cooling zones of the kiln on a train of rollers, made up of special-purpose steel or ceramic material and powered from a point outside the kiln. The useful effect of such kilns is that they may be run without the use of carriages and refractory burning tools, such transporting systems and burning tools for supporting the ceramic material having a great mass which has to be heated up by the kiln and then cooled down again in the cooling zone so that a large amount of the energy necessary for working the kiln is in fact wasted by such heating and not used by firing the ceramic structures.
In the case of such roller conveyor kilns firing the ceramic structures in a single layer and without the use of transporting systems and burning tools, not only is much less heat needed, but furthermore the firing operation itself takes place at a higher speed and more evenly and production cycles are very much shorter.
The cooling zone of such a kiln is normally designed for "quench" cooling, that is to say the necessary amounts of air are forced into the cooling zone to take effect on the ceramic structures after firing. However in such a kiln it is not possible to get round a shortcoming in this respect that--even if the firing zone is separated from the cooling zone by a curtain--the air from the cooling zone has an effect on the firing zone and, if the firing zone is run with a reducing atmosphere, this will be responsible for re-oxidation of the ceramic structures in the cooling zone and at least in the part of the firing zone which is next thereto. For stopping such re-oxidation of the ceramic structures in the cooling zone and a running back of such re-oxidation effects into the parts of the firing zone next to the cooling zone, it is possible (see German Offenlegungsschrift specification No. 2,824,367) for materials causing reduction in the cooling zone to be run into the kiln at a high pressure and at such a level and at such points in time, that, dependent on the excess still present in the kiln after part-reduction produced by putting in the amount of reducing agent, of the same is so small in amount that in the times between one input of reducing material and the next one, such excess may be burned, without any additional air being needed, by the neutral kiln atmosphere.
It will be clear to the reader that for running such a process a very complex control system is needed.